1. Field of Invention
The present invention generally relates to high intensity discharge (HID) lamps and more specifically to a high intensity strobe lamp assembly and method that produces strobe illumination without electromagnetic interference (EMI).
2. Brief Description of the Related Art
By way of background to the ensuing discussion of the related art to the present invention, and in connection with the description of the invention hereinafter, set out below is a glossary of relevant terminology.
As used herein, a “high intensity discharge (HID) lamp” means a lamp using a ballast to generate an electrical charge and to regulate voltage and current, which produces illumination when an arc of electrical energy passes across an arc gap, through gas contained in the lamp. Examples of HID lamps include, without limitation, metal halide lamps, compact metal halide lamps, pulse-start metal halide lamps, high pressure sodium lamps, white high pressure sodium lamps, mercury vapor lamps, and low-pressure sodium lamps.
The term “incandescent lamp,” as used herein, means a lamp that produces illumination by heating of an internal filament to generate radiant light.
The term “fluorescent lamp,” as used herein, means a lamp that uses phosphorus material to produce light when a gas is activated to energize the phosphorus material.
The term “harmful EMI,” as used herein, means electromagnetic radiation generated at a frequency≧1 MHz that produces undesirable interference with the operation of electrical/electronic equipment.
The term “continuous current,” as used herein in reference to an HID lamp, means an electrical current, e.g., a direct current (DC) or an alternating current (AC), which is uninterrupted across an arc discharge of a HID lamp throughout the powered operation of such lamp.
The term “strobe,” as used herein in reference to a lamp, means a lamp that produces intermittent discrete bursts of visible light (“strobing emissions”) in a cyclic and repetitive manner, wherein successive bursts are separated by periods of lamp operation with no visible light emission or with light emission that is of substantially lower intensity than the bursts of visible light (“non-strobing operation”).
The term “powered operation,” as used herein in reference to a strobe lamp, means electronic operation of the strobe lamp producing strobing emissions and non-strobing operation.
The strobing process has been conventionally carried out by cyclically and repetitively turning the power to a lamp on (with current flow to the lamp) and off (with zero currernt flow to the lamp), in an alternating and repetitive cycle.
Many types of lamps have the capability to be operated as strobe lamps. Such lamps include halogen, LED, incandescent, fluorescent and high intensity discharge (HID) lamps. HID lamps are particularly preferred for strobing applications, due to the higher energy efficiency of HID lamps (lumens emitted per watt of input power) than other lamps.
HID lamps, however, have a significant disadvantage in strobe operation. Once the arc discharge of the HID lamp is disrupted during turn-off of the current to the lamp, the lamp cannot restart without a significant time delay, which may in some cases be as long as one minute, unless a high voltage (e.g., on the order of 30 kV–50 kV) is applied to the lamp. When such high voltage is applied to a non-powered HID lamp, however, electromagnetic interference (EMI) signals are generated in the MHz and GHz range. Because most electronic devices operate in the MHz and GHz range, the EMI that is generated (in turn-on of the HID lamp by impressing high voltage thereon) can cause damage to or malfunction of electronic devices that are in proximity to the HID lamp EMI source.
The prior art metal halide strobe light systems that produce such deleterious EMI effects include the metal halide lamp strobe system of U.S. Pat. No. 6,501,231, wherein high voltage power, e.g., at 30 kV, is cyclically applied to the lamp to produce strobing action.
Such lamps as a result of their high-EMI output cannot be utilized in environments containing sensitive electrical/electronic equipment whose operation would be compromised by EMI exposure.
Environments of such type include aircraft environments in which HID strobe lamps as a result of their luminous intensity are highly desirable for exterior lighting of the aircraft, but in which sensitive avionics equipment are susceptible to malfunction and failure as a result of the deleterious EMI radiated by these lamps during their switch-on.
Efforts have been made in the prior art to reduce EMI from HID lamps by shielding approaches. For example, U.S. Pat. No. 5,530,634 describes a shielding arrangement in which an EMI-emissive area is covered with a metal foil. Shielding, however, increases the cost and complexity of the lamp system, is labor-intensive to install, and is susceptible to deterioration and mechanical failure, with consequent adverse effect on electrical/electronic equipment in the environment of the EMI-emitting HID lamp.
There is therefore a continuing need in the art for HID lamps that are free of deleterious EMI effects.